Exploring Nano-sized Fuel Additives

Over the last decade, fuel prices in the United States increased to more than $3 per gallon, prompting everyone from drivers to auto manufacturers and even lawmakers to look for ways to increase the fuel efficiency of vehicles. Some companies are claiming to have done exactly that, by manufacturing fuel additives made of nanometer-sized cerium particles.

These additives are meant not only to increase engine efficiency, but also to reduce the amount of soot that leaves the exhaust pipes of diesel trucks and buses. However, this may cover only part of the story, so EPA scientists are researching other effects the additives might have on overall air quality.

A team of EPA researchers, including Robert Willis, Michael Lewandowski, Jason Weinstein, and Prakash Bhave, are looking at the question of what cerium oxide does to vehicle emissions—and human health—by breaking the problem into several pieces. Willis, a measurement specialist, is examining the difference between ordinary diesel emissions and those from diesel mixed with nano-cerium additives. Bhave, who has worked extensively with atmospheric models, is focusing his efforts on predicting how cerium-induced emissions will change air quality.

“There are several possible effects that could result from adding nano-cerium to fuel,” Willis said. “One question we’re interested in answering is how it changes the composition of diesel exhaust.”

At room temperature, cerium nanoparticles form an extremely fine, inert powder, but these properties change at high temperatures found in combustion engines. There, nanoparticles help oxidize carbon at a lower temperature than is normally required to power a diesel engine. This helps improve fuel economy and decrease soot emissions. However, when nano-cerium is added to fuel, the exhaust from vehicles burning that fuel contains some cerium.

“Having cerium in the fuel can actually change all the pollutant levels,” Willis said.

Unfortunately, many of these pollutant levels can change for the worse. Lowering the amount of one emission-related chemical can significantly elevate the amount of another, and the newly increased level of that second pollutant might have unintended consequences. For example, the pollutant that is increased might react with other molecules in the atmosphere and create toxic byproducts in the air we breathe.

EPA scientists Robert Willis (left) and Prakash Bhave. On the screen behind them is an electron scanning microscope image of a common soot particle.

“Another question we’re trying to resolve is whether it’s a ‘nano’ problem,” Willis said. Because nanoparticles are very small, they can penetrate our respiratory tracts and enter our bloodstream in ways that larger particles cannot. However, it’s unclear whether cerium will leave the tailpipe in a nanoparticulate form. “If they attach to larger soot particles, they can’t go as far into the lung’s tissues,” he said. “So we’re trying to determine the form in which cerium will be emitted.”

The measurements Willis is making will figure into Bhave’s team’s efforts to model the way that nano-cerium additives might change the air to which we’re exposed. Chemicals and pollutants that leave tailpipes don’t enter a vacuum—once in the Earth’s atmosphere, they encounter numerous other particles with which they can combine.

For example, if cerium nanoparticles attach to micrometer-sized particles commonly found in the environment, the cerium could travel hundreds of miles away from the truck that emitted it and become ubiquitous in the atmosphere. If, on the other hand, nano-cerium exits the tailpipe on its own in a nanoparticulate form, it will quickly diffuse onto the surfaces of plants, trees, and buildings that are within a few hundred yards away. “In this case, one might need to investigate the effects of nano-cerium on plants and ecosystems in addition to the health of people living very near the roadways,” Bhave said.

Using sophisticated models of the atmosphere, Bhave can estimate the impact of a given set of emission changes.

“What if all the diesel vehicles across the United States were to start using nano-cerium fuel additives? By assembling the exhaust measurements from vehicles that have been tested with a cerium additive, we can generate a modified emissions scenario for input to our mathematical models,” he explained. “We run the model twice—once with present-day emission profiles, and a second time with the modified emissions scenario, and we can predict changes in various pollutant concentrations.”

The information and models that Willis, Bhave, and their partners generate will inform future research directions, including explorations of potential human health impacts, if necessary.

It’s not clear yet whether nano-cerium fuel additives will prove to be helpful, harmful, or benign, but Willis and Bhave’s study, which is expected to be completed in 2012, should provide some answers, and help inform future U.S. fuel additive regulations.